Chemical-mechanical polishing (CMP) is fast becoming an industry standard manufacturing process in semiconductor wafer fabrication. CMP is used to planarize various dielectric and conductive layers which are deposited on a wafer. Planarization is necessary to enable subsequent layers to be deposited and patterned without the underlying topography adversely affecting the patterning and deposition of these layers. In a typical CMP process, a wafer is held in a carrier and pressed against a rotating polishing pad in the presence of a slurry. A combination of the chemical and mechanical interaction between the semiconductor wafer, the slurry, and the polishing pad results in removal of the exposed films.
Despite the success of CMP, an ongoing problem with CMP is the ability to achieve uniform polishing across a wafer surface. The rate at which material is removed from the wafer varies radially from center to edge of the wafer. Typically, the center of the wafer polishes slower than the edges of the wafer. A fact which compounds this problem of polishing is that the wafer being polished is often not perfectly flat. For example, the wafer can be bowed to be either concave or convex, or can have more localized concavities or convexities. Factors which affect the shape of the wafer are the various thermal processes which the wafer has seen previously, and the types of films which are deposited onto the wafer.
Various solutions have been proposed to resolve the problems associated with non-uniform polishing. One proposed solution is to use a shaped carrier to hold the wafer during polishing. For example, the carrier is shaped to be concave or convex to match the shape of the wafer which is being polished. A problem with the use of the shaped carrier, however, is that the compensation is fixed. For example, a shaped carrier may work adequately for a convex wafer, but for a concave wafer the same carrier will actually degrade uniformity. Another solution to the problem is to shape the polishing platen rather than the wafer carrier. However, use of a shaped platen suffers from the same problem in that the shape of the platen is fixed while the shape of various wafers being polished will vary.
Yet another solution to the problem of polishing non-uniformity is the use of backside air. In using backside air, positive air pressure is applied to the backside of the wafer to cause the wafer to intentionally bow, thus increasing the contact area at the center of the wafer to the polishing pad. However, use of backside air likewise suffers from the fact that the amount of pressure being applied is constant regardless of whether the wafer being polished is concave, convex or flat.
Accordingly, there is a need in the semiconductor industry for a method for polishing semiconductor wafers which provides uniform polishing despite the incoming shape of the wafer being polished. Furthermore, it would be desirable for such a process to be easily integrated into existing manufacturing processes, and with minimal impact on manufacturing costs and wafer throughput.